The invention relates to a method for cleaning gases, particularly those generated by the pressure gasification of coal.
These gases usually cleaned by treatment in a scrubbing unit or scrubbing-and-cooling unit with a scrubbing liquid particularly water, which is drawn off from the scrubbing-and-cooling unit and is fed back again to the unit in a closed circuit. A small portion of the water is usually tapped off and the impurities contained therein, in particular tar, dust and salts, are separated. This small portion of the water can, if required be fed back again to the scrubbing-and-cooling unit.
Such a method is known. It works on the recirculation principle. The gas generated in the gasification unit is fed to the scrubbing-and-cooling unit under pressure and drawn off from the unit after cleaning and cooling, as cold gas. The heat required for evaporating a portion of the circulated water is principally used for cooling the gas. Water vapour is thus led off with the cold gas. During the operation, the circulated water is enriched, particularly with tar and dust and the cleaning efficiency of the water is therefore restricted. Fine-scrubbing or washing units are therefore required, through which the cold gas is led before it is used, e.g. is burnt in a gas turbine.
In order to maintain the cleaning efficiency of the water, a part of it is usually continuously drawn off and fed through a pressure relieving unit into a tar-separator, which works on the sedimentation principle. The cleaned water is fed out of the tar-separator into a storage unit, from which it is fed, together with fresh water, which replaces the quantity of water evaporated, back to the scrubbing and cooling unit.
The tar-separator must be made very large so that during the sedimentation process the retention times required for the separation of the small tar and dust content in the water, are attained. The storage unit must also be made sufficiently large so that the method and apparatus can be adapted to varying load conditions.
In order to adapt to varying load conditions, regulating units are provided which control the water level in the scrubbing-and-cooling unit and in the storage unit, and also control quantity of water being drawn off from the re-cycled water.
Since these regulating units have to be coupled to one another, a comparatively large expenditure on regulating equipment is necessary.
Another disadvantage of the known method is the loss of energy which occurs in cleaning the drawn-off portion of the water. In order to clean this portion, which in some cases may be from 1 to 7% of the quantity of the re-cycled water, its pressure must be reduced. The energy released in this pressure reduction process is therefore lost.
It is also known to clean gases by the "run-through" principle, which is also known under the name of the "Otto principle". In this method, only fresh water is fed to the scrubbing-and-cooling unit and the used water then flows away without cleaning. With this method admittedly the cleaning efficiency of the water is better than that achieved with the known recycling method, but this method is uneconomic in operation because of the large quantities of water which are required and which have to be cleaned up in order to protect the environment from pollution.
It is the object of the present invention to improve the gas cleaning efficiency achieved in the methods described above, and to reduce the cost of cleaning the water used in the cleaning process. Furthermore, the invention seeks to reduce the structural area required for the cleaning apparatus and to reduce the operating costs of the apparatus.